1. Technical Field
The present invention relates to a film-forming ink, a film-forming method, a method of manufacturing a light emitting element, a light emitting element, a light emitting device, and an electronic apparatus.
2. Related Art
Organic electroluminescence elements (organic EL elements) are light emitting elements having a structure in which at least one organic light emitting layer (light emitting layer) is interposed between an anode and a cathode. In such light emitting elements, by applying an electric field between a cathode and an anode, electrons are injected from the cathode and holes are injected from the anode, to a light emitting layer; the electrons and the holes are recombined in the light emitting layer to generate excitons; and when these excitons return to the ground state, the corresponding amount of energy is emitted as light.
In general, in the organic EL elements, a hole injection layer is provided on a anode and a hole transport layer or a light emitting layer is provided on the hole injection layer.
As a method of forming these layers (film-forming method), a method using a film-forming ink in which a film-forming material is dissolved or dispersed is known (for example, refer to JP-A-2008-77958).
In such a film-forming method, since patterning is possible without using photolithography, there is an advantage in that manufacturing processes are simple and the amount of raw materials used is small.
In general, as a constituent material of the respective layers of the organic EL elements, a π-conjugated compound is used. In the related art, the respective layers of the organic EL elements are formed using a film-forming ink in which the π-conjugated compound as a film-forming material is dissolved in an aromatic hydrocarbon solvent.
However, in the film-forming ink of the related art, the aromatic hydrocarbon solvent interacts with the film-forming material to cause π-π stacking, which leads to gelation and an increase in viscosity. Therefore, there is problem in that preservation stability is lacking.
In addition, in the film-forming ink of the related art, in addition to the interaction causing the above-described π-π stacking, since the solvent has a high boiling point, a desolvation property is low. As a result, there is a problem in that the characteristics of the obtained organic EL element deteriorate. When a film is formed using a liquid droplet discharge method, the above-described problems become significant because a force to prevent aggregation between π-conjugated compounds does not work unlike a spin coating method and thus the π-conjugated compounds easily aggregate.
In addition, when a light emitting layer is formed using a film-forming ink which contains a phosphorescent light emitting material formed of a metal complex, with a film-forming ink of the related art, the above-described problems relating to preservation stability and a deterioration in the characteristics of an organic EL element become significant. The reason is considered to be that a bond between a metal of a metal complex and a ligand is unstable due to effects of dissolved oxygen and water in a solvent.
In addition, in a film-forming ink of the related art using a non-aqueous solvent or dispersion medium, a contact angle with respect to a hole injection layer, which is formed using an aqueous solvent or dispersion medium, is large (that is, wettability is low). Therefore, there is a problem in that the film-forming ink does not wet and is not spread out on the hole injection layer sufficiently.
In order to avoid the above-described problems, JP-A-2008-77958 discloses a configuration of adding a surfactant to a solvent or a dispersion medium. However, this configuration is not practical because the surfactant remaining in the obtained film deteriorates the characteristics of a light emitting element.